Claude arrives in town, ready to join the army, and fight for his country. The army accepts him, and they train him to be a soldier. Meanwhile, he befriend a group of hippies (led by Berger) who don't agree with the war, but support Claude anyway.
A few months later, Berger persuades Claude to leave the army camp, and go and see his girlfriend. Berger would stay in the army camp, and pretend to be Claude during any roll-calls etc, so no-one would know Claude is missing.
But while Claude is out with his girlfriend... war strikes, and everyone is called to do their duty. But Berger hasn't been trained... he has no idea what to do in a war. But if he tells them he's not really Claude, then Claude will be court-marshelled. So Berger goes to war, still covering for Claude... and dies.